Method for applying a cermet electrode layer to a sintered electrolyte and electrochemical reactor

ABSTRACT

The invention relates to a method for applying a cermet electrode layer to a sintered electrolyte. This layer consists of a mixture of an ion-conducting oxide and a semiprecious metal oxide or precious metal oxide. According to the invention, the ion-conducting oxide is calcined without the presence of the (semi)precious metal oxide in order to provide for lateral electron conductivity, while the sintering of the mixture of the electrolyte takes place at as low a temperature as possible.

The present invention relates to a method according to theprecharacterizing clause of claim 1.

A method of this type, followed by the reduction of the metal oxides tothe metal is employed for fabricating electrodes in electrochemicalreactors. An example of this is given in the article "Characteristics ofSlurry-Coated Nickel Zirconia Cermet Anodes for Solid Oxide Fuel Cells"by Tasuya Kawada et. al. in J. Electrochem. Soc., Vol. 137, No. 10,October 1990, pp. 3042-3047. In this case, the metal oxide employed wasnickel oxide, and the oxygen ion-conducting oxide employed wasyttrium-stabilized zirconium oxide (YSZ). A mixture of nickel oxide andYSZ is calcined after mixing, is applied to the electrolyte in the formof a slurry and is then sintered, a reduction treatment finally takingplace to convert the metal oxide into metal.

For electrodes of this type, which are used, in particular, as plateanodes in solid-oxide fuel cells, the lateral electron conduction isimportant, in order to be able to take off current. Moreover, inconnection with promoting the electrochemical reaction, a highcatalytical activity is important. Because fuel cells of this type aregenerally operated at an elevated temperature (from 800° C.), it isimportant that the coefficient of expansion of the layer and of theelectrolyte which serves as the support of the anode are approximatelyequal in order to avoid, as far as possible, thermal stresses during theheating and cooling cycles. Finally it is important that no shrinkagearises during sintering.

The method described in the abovementioned publication by Kawada isparticularly suitable for fabricating small electrode. However, whenaiming for putting larger-scale fuel cells into practice, it isimportant likewise to be able to coat in this manner electrolytes havinga larger surface area. It was found that, when an electrolyte having alarger surface area was coated in this manner, problems arose as aresult of the considerable sintering shrinkage during sintering and ofthe lack of lateral conductance after reduction, so that the electrodethus fabricated will fail.

The German Offenlegungsschrift 2,852,638 discloses a method according tothe precharacterizing clause of claim 1. The starting point for thesensors fabricated according to that patent specification is formed bymetals or semiprecious metals. According to this GermanOffenlegungsschrift, a relatively high sintering temperature isemployed. The properties required of gas sensors are completelydifferent from the demands made of electrochemical cells. In the case ofa gas sensor, the determination of an electromotoric force between thegas to be analysed and a reference gas is the only thing that matters.In an electrochemical cell, the electrodes must be suitable for highcurrent densities in order to produce a sufficient output. In order toachieve such a high current density, relatively coarse ion-conductingoxides should be present and the metal or precious-metal particlesshould be as small as possible.

The object of the present invention is to provide a method by which itis possible to eliminate the drawbacks existing for gas sensors for usein electrochemical cells. This object is achieved in a method describedhereinabove having the characterizing features of claim 1.

It has been found that it is possible, by precalcining theion-conducting oxide, to adjust, independently of the metal oxide, theparticle size thereof. The higher the precalcination temperature, thelarger the particle size. By starting from oxides of precious metals orsemiprecious metals, simpler grinding is possible. The lower sinteringtemperature, compared to the prior art, prevents the oxide particles,after sintering, from adhering to one another in such a way that afterreduction lateral conductance is no longer possible.

The ion-conducting oxide may be a member of the crystal structure classof the perovskites and fluorites which can be formed from the transitionmetals, the rare earth metals and the alkaline earth metals. In the caseof the fluorites it is possible, in particular, to opt for zirconia,ceria and hafnia, doped with trivalent rare earth metal ions or divalentalkaline earth metal ions. In the case of the perovskites,ion-conducting cerates or zirconates may be chosen.

Preferably, the ion-conducting oxide comprises the yttriumoxide-stabilized zirconium oxide, and the (semi)precious metal oxidecomprises nickel oxide, the zirconium oxide preferably being stabilizedby means of 8 mol % of yttrium oxide. The calcination temperature forthe YSZ in such an embodiment is preferably between 1250° and 1600° C.,and the sintering temperature between 1200° and 1300° C.

The invention also relates to an anode for a solid oxide fuel cell,comprising an electrolyte which has been coated in the abovementionedmanner with a layer of material and in which the metal oxide has beenconverted into metal by reduction.

The invention will be explained below in more detail with reference toan example.

For the purpose of preparing an anode of a fuel cell, an electrolyteconsisting of a densely sintered ceramic material was coated with aslurry comprising NiO/8 mol % Y₂ O₃ -stabilized ZrO₂. After reduction,55% by volume of nickel was present, but it should be understood thatthe nickel percentage can be chosen over a wide range betweenapproximately 30 and 70% by volume. The slurry was produced by mixingnickel oxide powder with precalcined yttrium oxide-stabilized zirconiumoxide. This mixing took place in a ball mill device, in a solution ofbinder. This slurry was then cast onto the electrolyte, by means of theso called "tape casting" technique, and the anode from the electrolytewas sintered in air. Reduction then took place. Calcining of the YSZtook place independently (thus without the presence of NiO). Testsshowed that, depending on the calcination temperature, the particle sizeof the YSZ changed:

                  TABLE 1                                                         ______________________________________                                        Average particle size of 8YSZ was a function of the calcination               temperature and the average particle size of Ni as a function of the          sintering temperature.                                                        Tape code                                                                              8YSZ particle                                                                              Ni particle                                                                            8YSZ/Ni particle                               T.sub.pc T.sub.s                                                                       size (μm) size (μm)                                                                           size ratio                                     ______________________________________                                        1250/1200                                                                              0.48         1.2      0.4                                            1400/1300                                                                              17.8         2.4      7.4                                            1400/1400                                                                              17.8         3.2      5.5                                            1500/1200                                                                              10.4         1.2      8.7                                            1500/1300                                                                              10.4         1.7      6.1                                            1500/1400                                                                              10.4         2.0      5.2                                            ______________________________________                                         T.sub.pc = Precalcination temperature; T.sub.s = Sintering temperature.  

The particle size of the nickel oxide, measured under the sameconditions, was found to be 1-2 μm.

The mixture, consisting of nickel oxide and YSZ, applied to theelectrolyte, was then sintered at various sintering temperatures. Thesintering temperatures were 1200°, 1300°, 1400° and 1500° C.

The lateral electron conductivity was measured of the product formed,this being, as specified hereinabove, an important parameter fordetermining whether an electrode is satisfactory or not. This lateralelectron conductance was measured at 950° C., employing a so-called"four-probe" method which is described in more detail in L. Plomp, A.Booy, J. A. M. van Roosmalen and E. H. Pl. Cordfunke, Rev. Sci. Instrum.61, 1949 (1990).

The following results were observed:

                  TABLE 2                                                         ______________________________________                                        T.sub.c /T.sub.sinter                                                                      1200   1300       1400 1500                                      ______________________________________                                        -- as received                                                                             -      -          -    -                                         1250         +/-    +          +    -                                         1400         +      +          +    -                                         1550         +      +          +    -                                         ______________________________________                                         - = lateral electron conductance absent                                       + = lateral electron conductance present                                 

This table shows that good results are obtained if a relatively highcalcination temperature and a relatively low sintering temperature isemployed.

Comparison with Table 1 shows that, for the lateral electronconductance, it is likewise important to use YSZ particles having anaverage grain size greater than 1 μm. This could be explained withreference to the percolation theory, but it should be understood thatthe validity or otherwise of this theory does not affect the scope ofprotection of this application. This table shows, moreover, that nolateral electron conductance takes place above 1400° C.

Then the effect of the calcination temperature and the sinteringtemperature on the anode potential was determined with the aid of the"three-electrode" method. This involves measuring the voltage loss overthe anode (effective electrode area of 3 cm²) measured at a currentdensity of 100 mA/cm² and an operating temperature of 920° C. Theresults of this test are depicted in Table 3.

                  TABLE 3                                                         ______________________________________                                                8YSZ/Ni-                Anode losses (mV)                             Tapecode                                                                              particle R.sub.b   A.sub.ca                                                                           at 100 mA/cm.sup.2 and                        T.sub.pc /T.sub.s                                                                     size ratio                                                                             (Ohm)     (%)  920° C.                                ______________________________________                                        1250/1200                                                                             0.4      0.662     5.5  570                                           1400/1300                                                                             7.4      0.172     21.3 294                                           1400/1400                                                                             5.5      0.2403    15.3 303                                           1500/1200                                                                             8.7      0.1233    29.7 240                                           1500/1300                                                                             6.1      0.2556    14.3 321                                           1500/1400                                                                             5.2      0.5758    6.3  411                                           ______________________________________                                         T.sub.pc = Precalcination temperature; T.sub.s = Sintering temperature;       R.sub.b = Measured electrolyte resistance;                                    A.sub.ca = Effective crosssectional area.                                

Table 3 shows that the highest calcination temperature for YSZ and thelowest sintering temperature give an anode having the lowest voltageloss of the electrode. It is true, moreover, that an electrode of thistype has the best shrinkage behaviour, i.e. the lowest sinteringshrinkage during production.

Finally, the same method was used to determine the impedance from theelectrode-electrolyte interface by means of a "Solartron 1255 frequencyresponse analyser". Using a theoretical mode, it is possible todetermine, from these impedance data, the effective electrolyte surfacearea covered by active anode sites. The contribution of the electrodeimpedance and the electrolyte impedance can be separated in theimpedance spectrum. In general, the measured value of the electrolyteresistance R(B) does not agree with the expected (theoretical) value ofthe electrolyte resistance. This can, in part, be explained by the factthat a small fraction of the electrolyte surface area contributes to theconductivity, i.e. only that part which is covered by Ni particles fromthe anode microstructure. With the aid of the formula:

    A=ρ.sub.B *T.sub.B /R.sub.B                             1!

the apparent or active electrolyte surface area can be calculated as afraction of the total electrolyte surface area. In this formula, A isthe active surface area, T_(B) is the thickness of the electrolyte,R_(B) is the measured electrolyte resistance and ρ_(B) the resistivityof the electrolyte at 930° C. In Table 3, the A_(CS) values (as afraction of the total electrolyte surface area) are given as a functionof the precalcination temperature and the sintering temperature. TheA_(CS) values were determined for the number of active sites on theinterface anode/electrolyte.

From Table 3 it is likewise possible to conclude that the anodes withthe highest precalcination temperature and the lowest sinteringtemperature give the best results.

The only FIGURE depicts the effect of the ratio of the particle size onthe loss in anode voltage. This FIGURE also depicts the mutualrelationship of Ni and 8 YSZ.

Although the invention has been described hereinabove with reference toa preferred embodiment, it should be understood that numerousmodifications are possible thereof without leaving the scope ofprotection of the present invention as it is described in the claims.

We claim:
 1. Method for applying a cermet electrode layer to a sinteredelectrolyte, which layer at least comprises an oxide (A) of asemiprecious metal or precious metal and an oxide (B) which conductsions, in which method a slurry is formed of (A) and (B), the slurry isapplied to the electrolyte and the coated electrolyte is sintered,wherein (B) is calcined at a temperature between 1250° C. and 1600° C.prior to the slurry being formed with (A), the particle size of (B)becoming greater than that of (A), the sintering being carried out at atemperature between 1200° and 1300° C., and wherein, after sintering,the electrolyte provided with a layer is subjected to a reducingtreatment in order to convert (A) into metal.
 2. Method according toclaim 1, wherein (B) is chosen from ion-conducting oxides in the crystalstructure class of the fluorites and perovskites, it being possible touse, in the case of the fluorites, zirconia, ceria and hafnia, dopedwith trivalent rare earth metal ions or divalent alkaline earth metalions, and, in the case of the perovskites, ion-conducting cerates orzirconates.
 3. Method according to claim 1, wherein the metal of (B) ischosen from copper, nickel, cobalt, silver, gold, platinum, palladium,rhodium or ruthenium, iridium.
 4. Method according to claim 1, wherein(B) is yttrium oxide-stabilized zirconium oxide (YSZ) and (A) is nickeloxide.
 5. Method according to claim 4, wherein the layer applied to theelectrolyte comprises nickel, as well as zirconium oxide stabilized with8 mol % of yttrium oxide (YSZ).
 6. Method according to claim 5, whereinthe calcination temperature is approximately 1500° C.
 7. A ceramicelectrochemical reactor comprising a sintered electrolyte having acermet electrode layer having therein an oxide that conducts ions, thelayer being produced by the method of claim 1.